Lithography is a planographic printing process that utilizes printing plates having the image and non-image areas in the same plane. An offset lithographic printing press typically includes a plate cylinder carrying one or more printing plates. An inker applies ink to the printing plate utilizing one or more form rollers. The ink film on the form roller contacts the image areas on the printing plate, transferring the ink from the form roller to the printing plate. The inked image on the plate is then transferred to a blanket cylinder which transfers the image to the material to be printed, such as paper. The process is referred to as offset lithography because the printing plate does not print directly onto paper, but is first “offset” onto a rubber blanket (offset blanket) before forming a reproduction of the image on the material.
Chemical treatments are used to create areas of different surface energy on the printing plate such that the ink adheres to some areas and not others. The printing (image) areas are typically hydrophobic and ink receptive, while the non-printing areas are typically hydrophilic and water receptive. To achieve this effect, the plate can be damped with a water-based fountain solution (dampening solution) before it is inked. The fountain solution can be stored in a container and delivered to the printing plate by one or more chrome (dampening) rollers. The fountain solution typically forms a film on the water receptive areas of the plate, but forms tiny droplets on the water repellent areas. When a form roller is passed over the damped plate, it is unable to ink the non-printing areas covered by the film, but is able to push aside the droplets on the water repellent areas, coating those areas with ink.
During the printing process, the plate must be continuously treated with the fountain solution in order to maintain the thin film, thus maintaining the hydrophilic character of the non-image areas. The thin film of fountain solution prevents subsequent applications of ink from covering non-image areas of the plate. However, there is a tendency for ink to build up on the dampening roller and adhere to non-image areas of the printing plate surface when printing is carried out with repeated interruptions. This is referred to as “scumming” or “ink feedback” and results in the undesirable formation of spots and rings on the printed material.
Ink feedback (i.e., tinting) and excessive ink build-up (i.e., scumming) on the dampening rollers are common problems of high-speed web-offset lithographic printing. In many instances, printing presses experience several color changes in a single day, which can increase the amount of tinting and scumming. In such instances, the chrome rollers and printing plate must be cleansed thoroughly so that they will maintain the ability to be receptive to water and to reject ink. The passive (hydrophilic) chrome rollers are provided to improve “damping” control by minimizing tinting and scumming, but are not perfect in this regard. Good printing will result only if tinting and scumming are prevented.
Tinting and scumming have become more of a challenge since the switch to alcohol-free fountain solutions in conjunction with continuous-roll dampening systems that directly apply fountain solution to the plate without any connection to the ink rolls. The dampening roller can pick up ink from the plate and transfer the ink back to the metering roll via porous and ink-sensitive chrome rolls within the metering system. With no path back to the inker, the emulsified ink continues to build-up. While methods exist for preventing the accumulation of ink on the rollers, many of those methods are expensive, inconvenient and can increase printing time. To that end, a solution is desired that will prevent or at least alleviate tinting and scumming problems.
Because fountain solutions are water-based, there are many commercially available fountain solutions containing water-soluble additives, such as surfactants and rosins. However, the presence of additives in the fountain solution can be detrimental to the printing process. Since the fountain solution is continuously applied during the printing process, the actual amount of additive cannot be controlled, which can lead to defects in the printed materials. Alternatively, the additive can be incorporated into the ink composition rather than the fountain solution. Typically, inks used in the printing industry are oil-based inks, and the additives that are incorporated into the inks are oil-soluble additives (see, e.g., U.S. Pat. No. 5,417,749, where the additive is a surfactant such as lecithin or an alkylpolyglucoside). Even though the amount of additive can be controlled when added to the ink composition, printing problems still can arise. However, oil-based inks containing water-soluble additives such as alkyl thioether surfactants or highly alkoxylated rosins have not been disclosed in the prior art.
U.S. Pat. No. 5,203,926 discloses a formulation used to keep the rollers and printing plates clean and fresh so as to prevent ink from sticking to the rollers and printing plates. This is accomplished by using a cleansing solution compatible with fountain solutions used in the printing industry. However, this method requires the printing process to be stopped so that the printing plates and rollers can be wiped with a clean cloth before printing is resumed.
In an attempt to eliminate ink feedback and build-up, fountain solutions incorporating water-soluble additives, such as surfactants, have been developed. The additives provide important wetting and emulsification properties and are also used to replace alcohols in the fountain solution, which are flammable, toxic and irritating. U.S. Pat. Nos. 7,240,615; 5,607,816; and 4,854,969; and U.S. Patent Publication No. 2002/0083865 disclose fountain solutions that incorporate surfactants. The surfactants not only facilitate the dispersion of the hydrophobic imaging compositions in the fountain solution, but also reduce ink feedback and build-up.
Dampening systems with novel parts designed for reducing the amount of ink feedback have been developed to address the problem of ink feedback and build-up. U.S. Pat. No. 4,724,764 discloses a dampening system used for applying fluid to the plate cylinder of printing presses. This system includes a dampening form roller, a dampening fluid transfer roller, and a conventional metering mechanism for metering the dampening fluid. The system reduces the amount of ink feedback by ensuring that the speed of the roller carrying the dampening fluid is lower than the speed of the plate cylinder. U.S. Pat. No. 6,951,174 also describes a dampening system that reduces the amount of ink feedback and build-up. The system is a keyless inking system that employs a subtractive roller system that contacts the form rollers and removes excess ink and fountain solution from the form roller after printing. Utilizing these systems requires obtaining novel parts and equipment, which must then be interconnected to the lithographic press.
A need therefore exists for a non-mechanical means for reducing the problems associated with web-offset lithographic printing, such as reducing or eliminating ink feedback and excessive ink build-up that occur on non-printing plates during the lithographic printing process. Another need exists for an ink composition that eliminates ink feedback and build-up during the offset lithographic printing process, thus resulting in good and efficient printing.